Fluid bed processes have been employed for many years, and provide superior heat and mass transfer characteristics as contrasted with fixed bed processes. It is known. e.g., to produce synthesis gas (hydrogen and carbon monoxide) from low molecular weight hydrocarbons, primarily methane, reacted in the presence of steam and oxygen at high temperature within a fluidized bed of catalyst. e.g., nickel-on-alumina, or admixture of catalyst and solids diluents; the diluent being added to better control the heats of reaction. The thermodynamic efficiency of this process is quite high, but nonetheless fails to achieve as high a level of conversion of the hydrocarbons as would be desired due to emulsion phase back-mixing, and the low amount of bubble phase/emulsion phase mass transfer. The gaseous effluent from the fluidized bed reactor, in conducting such reaction, often contains some entrained solids; an effluent readily processed in a fluidized bed reactor though not so in a conventional fixed bed reactor which is all too readily plugged by the solids.